Exam 2 PSL 431 Questions with
Correct Answers
Photoreceptors are - ANSWER-- the sensory receptor cell of the visual system
- sense light
- 2 kinds: rods and cones
General structure of photoreceptors - ANSWER-- outer segment: specialized for
phototransduction, contains photopigments
- inner segment: acts as cell body with nucleus
- axon terminal: releases glutamate to bipolar cells to convey visual information
Rods - ANSWER-- enable night vision
- contain a lot of photopigment rhodopsin which allows us to see in gray
- many rods synapse onto a single bipolar neuron (convergence) so light detection is
more sensitive detection of even dim light
- amplify light better than cones
Cones - ANSWER-- angle high acuity & color vision
- 3 types: red blue and green
- most cones in fovea
- respond quickly to light stimuli, so detect quick flickering better than rods
- lower light sensitivity
Photoreceptor cells - ANSWER-- contain discs full of light-sensing photopigments
- outer segments of rods and cones are composed of plasma membrane and filled
with membrane discs
Disc membranes - ANSWER-- contain most proteins needed for phototransduction
- light absorbing photopigments that act like a GPCR
Photopigments - ANSWER-- two components:
1) opsin
2) retinal
Opsin - ANSWER-- protein embedded in disc membrane
- does not absorb light
- varies for rods and cones
Retinal - ANSWER-- light absorbing part
- derivative of Vitamin A
- can only be obtained from diet
- in rods and cones
- light causes change in conformation
,Phototransduction in rods (1) - ANSWER-- light activates rhodopsin (acts like a light-
gated GPCR)
- converts 11-cis retinal to all-trans retinal
- all-trans retinal no longer fits within rhodopsin binding pocket, causing opsin to
undergo conformational change
rods (2) - ANSWER-- rhodopsin activates transducin (heterotrimeric G-protein)
- rods and cones have an associated heterotrimeric G-protein called transducin
- conformational change causes a-subunit to exchange GDP for GTP
rods (3) - ANSWER-- transducin activates cGMP phosphodiesterase (PDE), effector
enzyme
- a subunit of transducin binds and activates the effector enzyme PDE
rods (4) - ANSWER-- activated PDE decreases cGMP concentration
- there is normally cGMP in a 'dark' cell
- light-induced activation of PDE causes breakdown of cGMP
- cGMP is the second messenger
- dark = high conc of cGMP
- light = low conc of cGMP
rods (5) - ANSWER-- cGMP-gated Na+ channels close
- dark: cGMP conc is high and keeps channel open, keeps cell depolarized
- light: cGMP conc is low so channel closes, hyperpolarization
rods (6a) dark - ANSWER-- high cGMP levels so cGMP-gated Na+ channel is open
causing graded depolarization (not AP)
- this causes v-gated Ca++ channels to open and mediate glutamate release into the
synaptic cleft
- in the dark rods are continually releasing glutamate onto the postsynaptic neuron
(bipolar cell)
rods (6b) light - ANSWER-- light-induced closure of cGMP-gated Na+ channels
causes graded hyperpolarization
- v-gated Ca++ channels close
- reduced Ca++ influx and vesicle release
- decreased release of glutamate to bipolar cell
Rods and cones - ANSWER-- do not fire AP
- respond to light with graded changes in Vm
Dark adaptation - ANSWER-- light breaks down the cGMP and there is a lag in
regeneration
- light causes PDE to break down the cGMP
- once PDE is inhibited it takes a few seconds for sufficient cGMP to be made in cell
- once cGMP levels raise it can open the cGMP-gated Na+ channel
Phototransduction: cones - ANSWER-- mostly same as rods, except for the
photopigment
, - retinal is the same in all rods and cones
- 3 cone subtypes of cones
S cones - ANSWER-- detect short wavelengths ie blue
M cones - ANSWER-- detect medium wavelengths ie green
L cones - ANSWER-- detect long wavelengths ie red
Anomolous trichromats - ANSWER-- have one weak photopigment
Deutroanomaly - ANSWER-- green cone is weak, most common form of color
blindness
Protanomoly - ANSWER-- red cone is weak
Deuteranopia - ANSWER-- loss of green opsin function
Protanopia - ANSWER-- loss of red opsin function
Bipolar cells - ANSWER-- receive synaptic transmission from photoreceptors
causing graded potentials (no AP)
- send message to ganglion cells
Ganglion cells - ANSWER-- fire APs
- axons compose the optic nerve
Optic chiasm - ANSWER-- where some axons cross
- where axons enter the brain
Why do ganglion axons cross? - ANSWER-- each retina divided by an imaginary
midline drawn through fovea
- nasal retine: half closest to nose
- temporal retina: closer to ear
- if brain recieves input from both eyes, crossed and uncrossed axons, it enables
binocular vision
Temporal retina - ANSWER-- fibers do not cross at optic chiasm
Nasal retina - ANSWER-- fibers do cross at optic chiasm
Primary cortex - ANSWER-- receives a specific sensory modality input and does
initial processing of it
All cortex is - ANSWER-- organized into columns of cells
- columnar organization allows for processing of multiple features of the stimulus
- enhances sensory perception
Lesion of the visual pathway - ANSWER-- causes blindness
Correct Answers
Photoreceptors are - ANSWER-- the sensory receptor cell of the visual system
- sense light
- 2 kinds: rods and cones
General structure of photoreceptors - ANSWER-- outer segment: specialized for
phototransduction, contains photopigments
- inner segment: acts as cell body with nucleus
- axon terminal: releases glutamate to bipolar cells to convey visual information
Rods - ANSWER-- enable night vision
- contain a lot of photopigment rhodopsin which allows us to see in gray
- many rods synapse onto a single bipolar neuron (convergence) so light detection is
more sensitive detection of even dim light
- amplify light better than cones
Cones - ANSWER-- angle high acuity & color vision
- 3 types: red blue and green
- most cones in fovea
- respond quickly to light stimuli, so detect quick flickering better than rods
- lower light sensitivity
Photoreceptor cells - ANSWER-- contain discs full of light-sensing photopigments
- outer segments of rods and cones are composed of plasma membrane and filled
with membrane discs
Disc membranes - ANSWER-- contain most proteins needed for phototransduction
- light absorbing photopigments that act like a GPCR
Photopigments - ANSWER-- two components:
1) opsin
2) retinal
Opsin - ANSWER-- protein embedded in disc membrane
- does not absorb light
- varies for rods and cones
Retinal - ANSWER-- light absorbing part
- derivative of Vitamin A
- can only be obtained from diet
- in rods and cones
- light causes change in conformation
,Phototransduction in rods (1) - ANSWER-- light activates rhodopsin (acts like a light-
gated GPCR)
- converts 11-cis retinal to all-trans retinal
- all-trans retinal no longer fits within rhodopsin binding pocket, causing opsin to
undergo conformational change
rods (2) - ANSWER-- rhodopsin activates transducin (heterotrimeric G-protein)
- rods and cones have an associated heterotrimeric G-protein called transducin
- conformational change causes a-subunit to exchange GDP for GTP
rods (3) - ANSWER-- transducin activates cGMP phosphodiesterase (PDE), effector
enzyme
- a subunit of transducin binds and activates the effector enzyme PDE
rods (4) - ANSWER-- activated PDE decreases cGMP concentration
- there is normally cGMP in a 'dark' cell
- light-induced activation of PDE causes breakdown of cGMP
- cGMP is the second messenger
- dark = high conc of cGMP
- light = low conc of cGMP
rods (5) - ANSWER-- cGMP-gated Na+ channels close
- dark: cGMP conc is high and keeps channel open, keeps cell depolarized
- light: cGMP conc is low so channel closes, hyperpolarization
rods (6a) dark - ANSWER-- high cGMP levels so cGMP-gated Na+ channel is open
causing graded depolarization (not AP)
- this causes v-gated Ca++ channels to open and mediate glutamate release into the
synaptic cleft
- in the dark rods are continually releasing glutamate onto the postsynaptic neuron
(bipolar cell)
rods (6b) light - ANSWER-- light-induced closure of cGMP-gated Na+ channels
causes graded hyperpolarization
- v-gated Ca++ channels close
- reduced Ca++ influx and vesicle release
- decreased release of glutamate to bipolar cell
Rods and cones - ANSWER-- do not fire AP
- respond to light with graded changes in Vm
Dark adaptation - ANSWER-- light breaks down the cGMP and there is a lag in
regeneration
- light causes PDE to break down the cGMP
- once PDE is inhibited it takes a few seconds for sufficient cGMP to be made in cell
- once cGMP levels raise it can open the cGMP-gated Na+ channel
Phototransduction: cones - ANSWER-- mostly same as rods, except for the
photopigment
, - retinal is the same in all rods and cones
- 3 cone subtypes of cones
S cones - ANSWER-- detect short wavelengths ie blue
M cones - ANSWER-- detect medium wavelengths ie green
L cones - ANSWER-- detect long wavelengths ie red
Anomolous trichromats - ANSWER-- have one weak photopigment
Deutroanomaly - ANSWER-- green cone is weak, most common form of color
blindness
Protanomoly - ANSWER-- red cone is weak
Deuteranopia - ANSWER-- loss of green opsin function
Protanopia - ANSWER-- loss of red opsin function
Bipolar cells - ANSWER-- receive synaptic transmission from photoreceptors
causing graded potentials (no AP)
- send message to ganglion cells
Ganglion cells - ANSWER-- fire APs
- axons compose the optic nerve
Optic chiasm - ANSWER-- where some axons cross
- where axons enter the brain
Why do ganglion axons cross? - ANSWER-- each retina divided by an imaginary
midline drawn through fovea
- nasal retine: half closest to nose
- temporal retina: closer to ear
- if brain recieves input from both eyes, crossed and uncrossed axons, it enables
binocular vision
Temporal retina - ANSWER-- fibers do not cross at optic chiasm
Nasal retina - ANSWER-- fibers do cross at optic chiasm
Primary cortex - ANSWER-- receives a specific sensory modality input and does
initial processing of it
All cortex is - ANSWER-- organized into columns of cells
- columnar organization allows for processing of multiple features of the stimulus
- enhances sensory perception
Lesion of the visual pathway - ANSWER-- causes blindness
